AAPG Bulletin, V. 84, No. 8 (August 2000), P. 1152-1172.

Geochemical Characterization of Natural Gas: A Physical Multivariable Approach and its Applications in Maturity and Migration Estimates¹

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved.
¹Manuscript received January 12, 1998; revised manuscript received December 21, 1998; final acceptance January 15, 2000.
²Division of Geology-Geochemistry, Institut Français du Pétrole, 1 and 4 Avenue de Bois Préau, 92852 Rueil Malmaison Cedex, France; e-mail: [email protected]
³Petrobras Research and Development Center - Cenpes, Cidade Universitária, Quadra 7, Ilha do Fundão, 21949-900, Rio de Janeiro, RJ, Brazil.
We would like to thank Petrobras for the authorization to publish these results. In the field, sampling and useful discussions were made possible by all the colleagues of Petrobras-Salvador and Petrobras-Saõ Mateus, and in particular Fernando Taboada Fontes and Roberto d'Avila. Gilberto Pereira da Silva did a lot for the best and most efficient gas sampling. We would like also to thank Caroline Sulzer for performing all the chemical and isotopic analyses, and Caroline Magnier and Elvyn Marshall for careful editing. John Curtis, Moses Chung, and Martin Schoell made critical reviews that improved the logic and the presentation of this paper.

ABSTRACT

Gas geochemistry has recently been shown to enhance information regarding the geological history of hydrocarbons. In this paper, graphical representations of physico-chemical processes affecting the chemical and isotopic signatures of natural gases are exemplified. These diagrams are based on experimental studies and the use of basic statistics to extract significant and synthetic parameters from the geochemical data. From 11 chemical and isotopic ratios, a statistical analysis (PCA) yields two very important parameters. The first parameter, using mainly the C₂₊ fraction of the gas, relates to maturity and the second parameter, involving the proportions and d¹³C values of methane, indicates mainly segregative migration. Positive values of the second parameter indicate that gases accumulated far from their source, whereas negative values correspond to residual gas pools after leakage of a part of the fluids. A tentative reconstruction of the gas history has been performed in two Brazilian basins: the Espirito Santo basin and part of the Recôncavo basin. The Espirito Santo basin is located on the passive continental margin of the Atlantic Ocean, and the Recôncavo basin corresponds to an intracontinental aborted rift. In both cases, the source rocks are mainly lacustrine, with thermal maturities ranging between the oil window and the beginning of the gas window. Results show that in the Recôncavo basin, a major fault (the Mata Catu fault) acts as a drain for hydrocarbon migration at the basin scale, associating a major isotopic fractionation to the gas migration with a clear correlation between isotope fractionation and the distance of migration. In the Espirito Santo basin, this segregation appears in the platform sediments to a lesser extent and is absent in the gas pools located in the paleocanyons filled with turbidites. This long-distance migration in the platform sediments suggests that a hydrocarbon kitchen is located offshore.